1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method which are capable of satisfying both excellent fixing property and excellent adhesion resistance even when using a developing roller and a supply roller each having a diameter as small as 12 mm or less.
2. Description of the Related Art
Dry development systems employed in electrophotography are of two types: a development system using a two-component developer composed of toner and carrier; and a development system using a one-component developer containing no carrier. In recent years, with increased demands for cost reduction and downsizing of low-end laser printers, attention has been focused on the latter development system using a one-component developer. In a development device using a one-component developer, such a process is widely employed that a thin-layer forming member is disposed facing a developing roller, a toner conveyed on a surface of the developing roller is pressed by the thin-layer forming member so that the thickness of a toner layer is controlled, and the toner is charged while passing between the thin-layer forming member and the developing roller.
In the above process, however, the thin layer forming member generates heat by friction caused between the thin layer forming member and the developing roller. To achieve further downsized image forming apparatus, it is effective to make developing rollers and supply rollers have smaller diameters. However, in association with downsizing of developing rollers and supply rollers, heat generated by the thin-layer forming member increases and some of a toner adheres to the developing roller.
For preventing the toner adhesion caused by frictional heat between the developing roller and the thin-layer forming member and preventing occurrence of streaks on printed recording media, for example, Japanese Patent No. 3,919,541 proposes an image forming method of using a process cartridge which includes at least a latent image bearing member having a diameter of 33 mm or less, a developer container, a toner bearing member having a diameter of 20 mm or less placed in contact with the latent image bearing member, a supply roller placed in contact with the toner bearing member, and a toner conveyance unit configured to convey toner to the supply roller, in which a ratio of the revolution speed of the toner conveyance unit to that of the supply roller is 0.1 to 0.5, and the amount of a toner coat (A) on the toner bearing member and the filling rate of toner (B) in the developer container satisfy an inequation, 0.9B≦A≦3B, and in the process cartridge, a one-component toner is used which contains at least a binder resin, a colorant, and a wax and which has a methanol wettability half value of 30% to 80%.
However, since a methanol wettability half value varies depending not only on an exposed wax on the toner surface but also on the type and the amount of external additives, it is difficult to improve fixing property and to eliminate problems with the thin-layer forming member by simply adjusting the methanol wettability half value.
Thus, at present desired are an image forming apparatus and an image forming method which are capable of satisfying both excellent fixing property and excellent adhesion resistance and of forming high-quality images, even when using a developing roller and a supply roller each having a diameter as small as 12 mm or less.